The present invention is directed in general to thermoforming, and in particular to a method and apparatus for thermoforming articles from a polar polymer sheet wherein the wall thickness of the articles can be accurately controlled to predetermined dimensions.
Thermoforming is a well-known method for fabricating articles such as drinking cups, blister packages, food containers, etc., which uses any suitable drawing or deep drawing technique, such as vacuum forming, pressure forming, plug-assist, mechanical matched die forming or twin sheet hollow part forming or any combination of such techniques. In all cases the sheet material is heated up to the softening temperature called, forming temperature, so that it can be physically worked, i.e. work-formed into the desired articles. For example, some of the forming temperatures of General Electric Company's polymer sheets are about 450.degree. F. to 500.degree. F. for Xenoy.RTM. blend of polycarbonate and polybutylene terephthalate, about 340.degree. F. to 400.degree. F. for Lexan.RTM. polycarbonate, about 520.degree. F. to 560.degree. F. for Ultem.RTM. polyetherimide, and about 280.degree. F. to 340.degree. F. for Cycolac.RTM. acrylonitrile butadiene styrene.
Although the thermoforming process has enjoyed wide success for fabricating shallow depth articles, difficulties are encountered in the fabrication of deep drawn containers or articles having thin walls. It has proven difficult to control the flow of the plastic sheet material as it is work-formed and hence to control the wall thickness of the articles being thermoformed. Precise control of the wall thickness at predetermined areas is desirable in order to provide strength to the article as required, as well as to reduce the cost of the material which is used.
In a typical thermoforming process, the sheet is heated uniformly prior to the work-forming step. During the subsequent work-forming step, the heated sheet is expanded into a die cavity by means of applied differential pressure, e.g. by applying vacuum pressure or positive pressure to one side of the sheet. In the process, portions of the sheet are stretched more than others, resulting, for example, in thin and flimsy bottom corners in a deep drawn container, while other parts of the container will be relatively thick in cross section. Thus, wall ruptures during the work-forming step are fairly common in such a situation.
A number of techniques have been devised to address the aforementioned problem. For example, by using what is designated as a "plug-assist" thermoforming process, a tapered metal plug is employed to effectively prestretch the sheet in selected areas prior to work-forming in the die in an effort to improve the final article wall thickness distribution. Such a process requires relatively complex machinery and extensive process controls to achieve the desired results.
Another known method of controlling the wall thicknesses is by heating certain areas of the sheet preferentially, sometimes referred to as zone heating. This can be done by conductive, convective or radiant heating. The principal heating technique in use today for thermoforming articles, typically uses resistance heaters, ceramic heaters, quartz lamps, or the like. To localize the heating effect, selected areas of the sheet may be screened from the heat source.
The preferential heating techniques discussed above have a number of disadvantages. Where radiant or convective heating is used, the heat is externally and diffusively introduced into the sheet through the sheet surfaces. Since the temperature distribution is transient in nature, it will change with time as the heat diffuses through the plastic sheet surface. This effect precludes the possibility of achieving a sharply defined temperature distribution within the sheet.
In the case of conductive preferential heating, the heating process is not only slow, but the surface of the sheet must be heated to a higher than necessary temperature for the core within to reach the required thermoforming temperature. The overheated surfaces may cause the sheet to sag or prestretch prior to its transfer from the heating station to the thermoforming station and produce unpredictable wall thicknesses in the thermoformed articles.
It should be pointed out that the aforementioned zone heating patterns, and the shapes of the plugs used in a "plug-assist" process, are generally determined by trial and error. As such, they are rarely optimal for a particular plastic sheet, or for a particular heating process. Thus, existing thermoforming techniques do not permit precise control and the wall thickness of the final article cannot be accurately predicted.